This self-separation is achieved through lateral epitaxial growth (ELO, Epitaxial Lateral Overgrowth) over a mask, wherein the mask is provided with openings (windows). In these windows, preferably a thin initial layer (or also starting layer) is uncovered, which had been grown on a substrate (such as, e.g., sapphire) in advance. Growth starts from these windows. After coalescence of the thus forming islands, a coherent/continuous semiconductor layer grows further. The tension between the starting substrate and the grown layer causes the self-separation of the semiconductor substrate to occur, which then can be removed from the reactor as a coherent wafer. Such a process is, for example, known from Applied Physics Letters, Vol. 85, No. 20, 15.11.2004, pp. 4630-4632.
Layered structures made of group III-nitrides (Ga, Al, In) form the basis for a multitude of modern devices for high frequency power electronics, e.g. communication systems based on HFETs (Heterojunction Field Effect Transistor), sensorics, radiation-resistant air space electronics and for opto-electronics, e.g. UV-, blue and white light-emitting diodes (LEDs) and blue laser diodes for illumination, printing, display, memory and communication appliances as well as medical applications. Such layers are typically manufactured by means of metal organic vapour phase epitaxy (MOVPE) as well as by means of molecular beam epitaxy (MBE) on a starting substrate.
An ideal starting substrate shall belong to the same material system as the layers grown thereon, for example a GaN substrate. Thereby, requirements for a low-defect growth, i.e. a sufficiently good to perfect lattice mismatching (homoepitaxy) and a conformance of the thermal expansion coefficients are provided for in advance. Depending on the application, a doping is advantageous, which renders the substrate n-conducting, semi-isolating or p-conducting.
Contrary to other semiconductors, e.g. silicon (Si) and gallium arsenide (GaAs), to date the manufacture of GaN crystals having diameters of 2 inch or more by means of conventional single crystal growth has been unsuccessful. Conventional processes such as growth from the melt under high pressures and temperatures (HPSG—high pressure solution growth) result in crystal plates in the dimension of cm2 only. Up to now, growth by means of sublimation is unsuccessful, either. To date, the layered structures are therefore mostly epitaxially grown on foreign substrates such as, e.g., sapphire and silicon carbide (SiC) (heteroepitaxy). This is disadvantageous, e.g. with respect to the achievable dislocation densities and the warping resulting from different lattice constants and differences in thermal expansion. This warping partly also results in problems in the subsequent processing, because e.g. transfer of structures by means of photolithographic processes is limited on warped wafers in terms of resolution.
Therefore, attempts are made to produce GaN starting substrates which are grown on an initial starting substrate and separated therefrom.
U.S. Pat. No. 6,740,604 describes a process, wherein after the growth of a GaN layer on a starting substrate, this layer is separated in a subsequent process by means of laser irradiation. Thereby, a further process step is necessary, which is laborious and limited in yield for larger areas. Furthermore, this process does not solve the problem of warping, since the package consisting of GaN-layer and -substrate is cooled from growth temperature to room temperature. The warping of the layer induced by the thermal misalignment partly maintains still after separation from the starting substrate.
U.S. Pat. No. 6,413,627 describes a process applied on a GaAs starting substrate. Here, GaN is grown on both sides of a GaAs substrate being structured with a dielectric mask. This process requires a laborious etching process for removing the GaAs substrate, which, moreover, is toxic. Only one of both GaN layers can respectively be used, and a specific apparatus is necessary for dual sided growth.
Oshima et al. (Y. Oshima, T. Eri, N. Shibata, H. Sunakawa, K. Kobayashi, T. Ichihashi, A. Usui, Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation, Jpn. J. Appl. Phys. 42, L1 (2003)) describe a process, wherein a porous layer of TiN is deposited on a GaN initial layer, wherein growth of GaN starts out of the pores. However, it is unclear how the porosity of this TiN layer can be controlled, and whether this process can be performed in a reproducible manner. In the process proposed, the thick GaN layer is separated from the initial substrate by external forces, which requires an additional process step as well as an apparatus necessary therefore.
The process of lateral epitaxial overgrowth (ELO or ELOG) is known from WO 99/20816 as a possibility of reducing defects. Dielectric mask materials for the ELOG process are described therein. This process however does not avoid warping of the package consisting of starting substrate and GaN layer.
From DE 100 11 876 A1, the use of a metallic mask is known, which consists of tungsten in this case. Both publications deal with reducing defect density by the ELOG process. The formation of a free-standing substrate is not an object of these works.
Furthermore, a process for growing GaAs is known from U.S. Pat. No. 4,868,633, wherein not a lateral overgrowth of a multitude of column-shaped initial regions, but—contrary to that—a growth of a limited region is sought. A lateral overgrowth shall even be avoided. Therefore, a WSi:Zn mask is used, wherein the WSi:Zn mask shall avoid a lateral overgrowth. In FIGS. 1 and 2 of U.S. Pat. No. 4,868,633 (and the associated text), it is investigated how GaAs can be grown in the region of the column and its surrounding. When the concentration of W in the uppermost layer of a WSi mask is high, an island formation is suppressed. When the concentration of Si in the uppermost layer of the WSi mask however is high, a layer made of polycrystalline GaAs can be formed on the WSi mask. Since the formation of a GaAs layer on the WSi mask shall be avoided, the WSi mask is doped with zinc.
Furthermore, the use of WNx masks for the manufacture/treatment of GaN layers is published in Materials Science and Engineering: B, Vol. 82, No. 1, 22 May 2001, pp. 62-63 (3) (Abstract), wherein the WNx mask is used instead of a W mask, in order to avoid separation of a GaN layer.
The U.S. Pat. No. 6,146,457 describes a process, wherein a semiconductor layer is deposited by means of vapour phase epitaxy on a growth support consisting of a Si-, SiC- or sapphire-substrate and a thin intermediate layer, wherein defects are caused in the substrate but not in the epitaxial layer in the subsequent cooling due to different thermal expansion coefficients of semiconductor layer and substrate, thereby obtaining a highly qualitative semiconductor layer. In claim 3, silicon oxide, silicon nitride or silicon carbide are explicitly mentioned as possible materials for the intermediate layer. According to claim 4, the intermediate layer may also be structured, while the ELOG process for reducing defect density is neither mentioned nor described. Moreover, neither in the claims nor in the embodiments, a free-standing semiconductor substrate as a result of the process is mentioned.
Therefore, the object of the present invention is to provide a process for the manufacture of a free-standing (i.e. non-bound to a substrate) semiconductor layer, preferably made of gallium nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN) or indium gallium nitride (InGaN), which needs process steps as few as possible and which furthermore enables the manufacture of a planar, but not or only slightly warped or bowed semiconductor layer. Furthermore, a free-standing substrate shall be provided which is obtainable at low costs and which has a very good planarity.
These objects are solved according to the present invention by subject-matter with the features of the independent claims. Preferred embodiments are set forth in the subclaims.
The process according to the present invention for the manufacture of a semiconductor substrate is characterized by the following process steps:                providing a starting substrate,        forming a mask layer with a multitude of openings on the initial substrate,        growth of at least one semiconductor substrate, wherein the mask layer is laterally overgrown by at least one semiconductor material, and subsequently        cooling the starting substrate, the mask layer and the semiconductor substrate, wherein the material for forming the mask layer at least partially consists of tungsten silicide nitride or tungsten silicide,        separation of semiconductor substrate and starting substrate already during the growth or not before cooling, so that a semiconductor substrate is obtained free-standing, and wherein the semiconductor substrate contains at least one nitride compound semiconductor.        
Preferably, the material for forming the mask layer consists completely of tungsten silicide nitride or completely of tungsten silicide. Preferably, the mask layer of tungsten silicide nitride or of tungsten silicide is not doped with other substances.
Tungsten silicide nitride is particularly preferred. Preferably, a continuous initial layer is grown on the substrate before forming the mask layer, and the mask layer is deposited on the initial layer. Alternatively, it is possible to deposit the mask layer directly on the substrate without an initial layer, which is beneficial particularly for SiC substrates.
The structured mask layer is prepared preferably by depositing a continuous mask layer by means of sputtering or vapour phase deposition and subsequently introducing a multitude of openings. The openings are preferably introduced into the mask layer dry-chemically by means of plasma etching. Alternatively, wet-chemical etching or a lift-off process is possible as well.
On the thus structured mask layer, starting from the openings, at least a first semiconductor layer—the coalescence layer—made of semiconductor material is grown, which completely covers the mask material and forms a continuous layer.
The initial layer preferably contains a nitride compound semiconductor, particularly preferably a nitride compound of elements of the third and/or fifth main group, particularly preferably GaN, AlN, AlGaN, InN, InGaN, AlInN or AlInGaN.
On the first semiconductor layer, a further semiconductor layer may be deposited, preferably in the same growth process. The thickness is preferably more than 50 μm, more preferably more than 200 μm. Alternatively, the first semiconductor layer may also be produced in a first growth process and overgrown in a second growth process, over the whole area, by a second semiconductor layer. Preferably, also this second semiconductor layer contains a nitride compound semiconductor, particularly preferably a nitride compound of elements of the third and/or fifth main group, particularly preferably GaN, AlN, InN, InGaN, AlGaN, AlInN or AlInGaN.
Preferably, the starting substrate contains silicon carbide or sapphire. Preferably, on the thus produced semiconductor layer, further semiconductor layers, preferably containing a nitride compound semiconductor, or a metallic contact are placed for forming an electronic or opto-electronic device.
The free-standing semiconductor substrate is preferably formed by crack formation at the interface to the starting substrate as well as in the regions of the semiconductor substrate within the openings of the mask, based on the tensile stress during the growth or the different thermal expansion coefficients of the starting substrate and the at least one semiconductor substrate during cooling.
The semiconductor substrate according to the invention for producing electronic or opto-electronic devices comprises a nitride compound semiconductor (preferably GaN, AlN, AlGaN, InN, InGaN, AlInN, or AlInGaN), wherein the semiconductor substrate according to the invention comprises traces of tungsten silicide nitride or traces of tungsten silicide or traces of silicon and tungsten. The concentration of the traces depends on the detection limit for such residues after separation. They amount preferably to more than 1015 atoms per cm3. However, it can not be excluded that a semiconductor substrate can be produced by the process according to the invention, where the said traces cannot be detected.
The process enables the manufacture of low-defect, free-standing GaN wafers, which self-separate from the starting substrate. This separation may occur already during the growth due to tensile stresses of the grown semiconductor layer increasing with increasing layer thickness, or yet at the latest by the different thermal expansion coefficients during the cooling from a growth temperature. It has been found that such a separation occurs particularly when using tungsten silicide nitride or tungsten silicide as the mask material. Therefore, an additional technological step for separation can be dispensed with. Due to separation already at high temperatures, the GaN wafers have only a low or no warping or bowing, which is advantageous for the further processing.
Therefore, the mask layer according to the invention for the manufacture of a semiconductor substrate consists at least partially of tungsten silicide nitride. Preferably, it consists completely of tungsten silicide nitride. Preferably, the mask layer made of tungsten silicide nitride is not doped with other substances.
Preferably, a suitable initial layer is used, on which the epitaxial growth of the semiconductor layer(s) (for example of GaN) is possible. This initial layer preferably consists of a some μm thick GaN layer, which had been deposited heteroepitaxially on a starting substrate. As the starting substrate for the growth of GaN, sapphire, SiC, Si and GaAs have already been demonstrated. As for the processes for preparing the initial layer, any technology can be used which deposits a closed GaN layer on a starting substrate. Here, MOVPE, HVPE and MBE are widely used processes. Alternatively, the initial layer may consist of AlN, AlGaN, InN, InGaN, AlInN or AlInGaN. Preferably, it consists of the material of the subsequently deposited first semiconductor layer—the coalescence layer. When growing on SiC, this start/initial layer is preferably omitted.
When using an initial layer, a thin mask layer is deposited, which consists at least partially of tungsten silicide (WSi) or tungsten silicide nitride (WSiN). Without an initial layer, the mask layer would be deposited directly on the substrate. ELOG-masks typically have thicknesses between 50 and 200 nm. The deposited WSi- or preferably WSiN-layer is lithographically structured, and windows are opened by wet- or dry-chemical etching processes, in which the initial layer or the substrate (SiC-substrate) is exposed. For structuring, other processes can be contemplated as well, such as a so-called lift-off process. Such structuring processes commonly used in the semiconductor technology enable a defined and reproducible preparation of openings in the mask layers. Preferably, these openings are largely provided periodically, have a circular or polygonal geometry, or consist of parallel disposed stripes.
The use of a porous WSiN layer without a subsequent step for structuring is also possible. Here, reproducibility of the result of the process however is to be ensured.
On the masked initial layer or the masked substrate, an at least 50 μm thick GaN/AlGaN-layer (or another nitride semiconductor layer) is grown subsequently by means of vapour phase epitaxy. Here, the material grows vertically out of the windows and laterally over the mask, until the growth fronts coalesce and form a flat layer again. According to the invention, this first semiconductor layer does not adhere to the described mask layer made of WSiN, which is an essential prerequisite for the subsequent self-separation. The joint to the starting substrate is formed exclusively by the opened windows (openings) in this mask layer.
When selecting SiC as starting substrate, the deposition of a GaN initial layer can be omitted. The WSiN mask layer is deposited directly onto the starting substrate and structured. Subsequently, a cohesive first semiconductor layer, the coalescence layer, made of GaN, AlN or AlGaN or another nitride semiconductor is grown by means of ELO. Immediately subsequent in the same epitaxy process, or alternatively in another epitaxy process, a second, thick semiconductor layer of a nitride semiconductor, preferably having a thickness greater than 50 μm, can be grown on the whole area on this coalescence layer.
Suitably choosing the mask geometry, the deposition parameters and the process control, the grown second thick semiconductor layer, together with the first semiconductor layer—the coalescence layer—separates on the whole area from the starting substrate, and a free-standing wafer of e.g. 2 inch diameter is formed. This wafer can then be used for growing structured layers for devices, or as a seed for growing volume crystals made of GaN, AlN or AlGaN or another nitride semiconductor, wherein optionally process steps for smoothing the surfaces (polishing, etching) may still be carried out.